A major limitation in finding more effective cancer therapies is the identification and exploitation of novel targets for rational drug design. Recent and provocative evidence implicates a novel form of post-transcriptional gene regulation involving 3? RNA uridylation mediated by Terminal Uridylyl Transferases (TUTases) as a critical gene regulator and driver of tumorigenesis. These data indicate that dysregulated TUTase activity alone and in concert with the onco-fetal LIN28/let-7 pathway are hallmarks of poor prognosis in multiple cancer types. Approximately 15% of all cancers reactivate oncogenic LIN28 expression (either LIN28A or LIN28B) and these cancers are typically characterized by poor prognosis. Functionally, LIN28 directly blocks the biogenesis of the tumor suppressor let-7 microRNA family. Recently, we discovered the mechanism behind this blockade. Notably, the TUTase ZCCHC11 through an association with LIN28A inactivates let-7 by preventing the conversion of precursor let-7 into its mature, tumor suppressor form. In addition, overexpression of the TUTase ZCCHC11 is implicated in poor prognosis breast cancer independently of the LIN28/let-7 pathway. Strikingly, a recent report has shown that many polyadenylated mRNAs are frequently uridylated at their 3? ends in a gene-specific manner and targeted for degradation; however, this observation?s relevance to cancer is currently unknown. Therefore, our research has three major objectives. The first goal is to develop novel preclinical mouse models for LIN28-positive breast cancers. These mouse models will elucidate how LIN28 modifies disease progression and will be an invaluable resource to evaluate new drugs targeted to this cancer type. The second goal is to determine whether organism-wide loss of Zcchc11 in adult mice has phenotypic consequences using a conditional mouse knockout. Our published work on conditional inactivation of either Lin28A or Lin28B in adult mice demonstrate that both genes are dispensable in adults, suggesting that inhibiting either gene is unlikely to cause deleterious side effects in patients. The last goal is to determine if loss of the TUTase Zcchc11 inhibits Lin28A+/Her2+ and Lin28A-/Her2+ mouse mammary tumors. This research will be the first proof-of-principle that TUTase loss affects tumors in an immunocompetent mammal and addresses possible side-effects in cancer patients treated with specific inhibitors of the TUTase ZCCHC11. This work lays the foundation for new fields of cancer investigations by dramatically shifting the current paradigm to include TUTases as cancer gene regulators and explores the feasibility of a novel drug class targeted at inhibition of LIN28 or TUTases. In addition to cancer, our investigation of regulators of the LIN28/let-7 pathway holds significance for stem cell biology, tissue repair, fertility, growth, glucose metabolism, Type 2 diabetes, and the generation of induced pluripotent stem cells. Moreover, our genome-wide analysis of 3? RNA uridylation has implications for other human diseases where dysfunctional RNA turnover contributes to pathology such as multiple myeloma and Perlman syndrome.